This invention relates to current sources and more particularly to monolithic integrated current sources for providing source current to multiple load circuitry.
A contemporary current source utilized in monolithic integrated circuits for providing multiple output currents is such a source utilized in the MLM-124 Operational Amplifier manufactured by Motorola, Inc., for example. A comparative schematic illustrating this current source is disclosed in FIG. 1 of the subject specification labelled as Prior Art. As shown, this circuit comprises an input transistor and at least first and second sourcing transistors. A detailed discussion of the operation and significant problems related to this current source is described within the subject specification.
Basicially, this prior art current source, when manufactured in integrated circuit form suffers from two disadvantages. First, if for some reason, one of the output sourcing transistors should become saturated, the matching of current ratios between the input transistor and output transistors is imbalanced and current output from the remaining source transistor does not remain in the correct ratio with regards to the input current. Second, a parasitic transistor may be formed between the substrate of the integrated circuit with the collector-base junction of the saturated sourcing transistor. This parasitic transistor allows current to be injected into the substrate. Thus, current is not only wasted but more power is consumed in the integrated circuit. Both of the aforementioned problems are undesirable.
Thus, a need exists for an improved current source incorporating the features of the prior art and having protection against saturation of one or more of the output sourcing devices.